The sea is potentially full of undersea military threats, and that makes it daunting to detect them all using hardware. The scale and cost would be utterly impractical. DARPA, however, has a potential workaround: make sea life do the work. It recently launched a Persistent Aquatic Living Sensors program (yes, that shortens to PALS) that will study the viability of using both natural and modified sea organisms to detect underwater vehicles. If the US military can translate the biological responses of this sea life into usable data, it could warn ships without needing conspicuous, expensive hardware -- the fish and plants would be enough.